1. Field of the Invention
The present invention is generally related to the field of integrated circuits, and more particularly related to a method for avoiding excessive local heating that creates electromigration in the metallic interconnects of an integrated circuit device.
2. Background of the Invention
Metallic interconnections of integrated circuit (IC) devices are subject to a degradation phenomena known as electromigration (EM). EM is caused by the electronic interactions of the metal atoms and ions in the conductors of an IC device. EM effects become more prominent as IC feature sizes decrease and as IC frequencies and current densities increase.
EM in IC devices occurs due to direct current flow. High direct current density in an IC device causes atoms and ions in the conductors of the device to move in the opposite direction of the direct current flow. In particular, when high direct current densities pass through thin conductors, metal ions accumulate in some regions and voids form in other regions of the conductors. The accumulation of metal ions may result in a short circuit to adjacent conductors and the voids may result in an open-circuit condition. However, if the current density can be kept below a predetermined EM threshold, EM can be rendered negligible for the life of any particular IC device. Therefore, EM due to direct current flow in IC devices is a major concern with respect to the potential for device failures and the overall reliability of the device.
IC devices may also have alternating current flow. The alternating current density in an IC device that results from alternating current flow causes atoms and ions in the conductors of the device to first move in one direction and then move in the opposite direction, back to their original positions. A plurality of conductors with alternating current flow are defined as a signal net. In contrast to conductors with direct current flow, conductors with alternating current flow do not directly cause EM problems. However, conductors with alternating current flow do use power and generate heat. Since EM is very sensitive to the temperature of the conductors, it is often necessary to limit the temperature increase of the conductors in IC devices that results from the heating due to alternating current flow. Therefore, the alternating current flow in a conductor does have an impact on EM because the heating due conductors with alternating current may increase the overall temperature of the IC device by heating up neighboring conductors with direct current flow.
As noted above, EM effects also become more prominent as IC feature size decreases. To counteract this effect, background art methods for controlling EM used wider conductor widths for an entire IC wiring network affected by EM. However, since EM problems become less severe as one moves away from a current source pin and toward each of the current sink pins of a wiring network, wider conductor widths are typically not required for the entire IC wiring network. Often, only a small segment of the IC wiring network needs the wider conductor width to eliminate EM problems for the entire IC wiring network. Therefore, these background art methods that use wider conductors throughout the IC wiring network often wastes valuable space on the IC device.
Other background art methods provide EM control by setting limits on the power dissipated in conductors with alternating current flow. In these background art methods adjacent conductors with direct current flow are only allowed to be heated by a maximum temperature difference ΔTMAX in order to maintain the reliability of the IC device. In particular, to limit the heat generated as a result of the temperature difference ΔT caused by alternating current flow in adjacent conductors, a maximum root-mean-square (RMS) current limit (IRMS) is set for all conductors with alternating current flow adjacent to a conductor with direct current flow. The maximum current limit is set by: (1) considering the minimum distance between conductors with alternating current flow and conductors with direct current flow; and (2) the maximum temperature difference Δ-TMAX that maintains the reliability of the IC device. However, using this type of worst-case “minimum distance-between-conductors” approach to determine space between conductors also wastes valuable space on the IC device.
Therefore, there is a need to limit heating due to alternating current flow in an IC device by determining a power limit that provides device reliability due to excellent EM control and efficiently uses the space available on the IC device.